104 
THE PRINCIPAL FLOOR. 
whereby r the powder is rendered perfectly coherent. From the 
blocks thus formed slices are sawn, from which pencils of the 
best quality, and entirely free from grit, are formed. 
In addition to the Cumberland graphite, specimens of the 
mineral are exhibited from Siberia, Ceylon, and Canada, as well 
as from several other localities. Ceylon yields most of the 
plumbago used for crucible manufacture. 
The remainder of this Case is occupied with some of the forms 
of coal, which are rich in carbon. The mother of coal or mineral 
charcoal , which often occurs between the layers of bituminous 
coal, is a soft fibrous material, readily soiling the fingers, and 
containing between 75 and 83 per cent, of carbon. The varieties 
of coke represent the carbonaceous constituents of coal as 
obtained artificially by processes of destructive distillation. 
There are also here some samples of the so-called “ graphite,” or 
gas-carbon , found lining the interior of gas retorts, and valued 
as a conductor of electricity. 
The varieties of coal, occupying part of this Case and of the 
next, form a regular series, commencing with the non-bituminous 
stone-coal or anthracite, and passing thence through the ordinary 
bituminous coals to the recently-formed lignite or brown coal, 
in which the original structure of the wood is frequently 
retained. 
Anthracite always contains a large per-centage of carbon, 
frequently upwards of 90 per cent., and but a. small proportion 
of volatile constituents. As the anthracites become softer they 
are distinguished as free burning or steam coals, and these may 
pass into the ordinary bituminous varieties, as in the South 
Wales coal-field. ( See Case of Coals in the Model Room B.) 
The crystallised ” coal exhibits a very peculiar structure, 
known as cone-in-cone , whilst in the peacock coal we have a 
remarkable example of superficial iridescence. 
Case C. — Coal, Jet, Amber, &c. 
A large part of this Case is occupied by a continuation of the 
series of coals, and by various hydrocarbons more or less closely 
related to this series. 
Cannel is usually regarded as a compact variety of coal which 
burns readily, with a clear flame like that of a candle, whence 
its name, candle, or, in local patois, cannel coal. Large 
quantities are raised near Wigan in Lancashire, and at 
Lesmahagow, about twenty miles from Glasgow. A fine block 
of Lancashire cannel stands in the Lower Hall, No. 53. The 
Albertite from Nova Scotia, valuable for gas-making, appears 
to be an asphaltic variety of cannel. 
Lignite, wood coal , or brown coed, which frequently retains 
the original texture of wood, usually occurs in deposits of 
tertiary age. In Germany, and in many other parts of the 
continent, lignite is extensively worked, but in this country it is 
